[RFC,v2,0/3] Allow sk_lookup UDP return traffic to egress when setting src port/address.

Message

Tiago Lam Sept. 20, 2024, 5:02 p.m. UTC

Currently, sk_lookup allows an ebpf program to run on the ingress socket
lookup path, and accept traffic not only on a range of addresses, but
also on a range of ports. At Cloudflare we use sk_lookup for two main
cases:
1. Sharing a single port between multiple services - i.e. two services
   (or more) use disjoint IP ranges but share the same port;
2. Receiving traffic on all ports - i.e. a service which accepts traffic
   on specific IP ranges but any port [1].

However, one main challenge we face while using sk_lookup for these use
cases is how to source return UDP traffic:
- On point 1. above, sometimes this range of addresses are not local
  (i.e. there's no local routes for these in the server), which means we
  need IP_TRANSPARENT set to be able to egress traffic from addresses
  we've received traffic on (or simply IP_FREEBIND in the case of IPv6);
- And on point 2. above, allowing traffic to a range of ports means a
  service could get traffic on multiple ports, but currently there's no
  way to set the source UDP port egress traffic should be sourced from -
  it's possible to receive the original destination port using the
  IP_ORIGDSTADDR ancilliary message in recvmsg, but not set it in
  sendmsg.

Both of these limitations can be worked around, but in a sub-optimal
way. Using IP_TRANSPARENT, for instance, requires special privileges.
And while one could use UDP connected sockets to send return traffic,
creating a connected socket for each different address a UDP traffic is
received on does have performance implications.

Given sk_lookup allows services to accept traffic on a range of
addresses or ports, it seems sensible to also allow return traffic to
proceed through as well, without needing extra configurations / set ups.

This patch sets out to fix both of this issues by:
1. Allowing users to set the src address/port egress traffic should be
   sent from, when calling sendmsg();
2. Validating that this egress traffic comes from a socket that matches
   an ingress socket in sk_lookup.
   - If it does, traffic is allowed to proceed;
   - Otherwise it falls back to the regular egress path.

The downsides to this is that this runs on the egress hot path, although
this work tries to minimise its impact by only performing the reverse
socket lookup when necessary (i.e. only when the src address/port are
modified). Further performance measurements are to be taken, but we're
reaching out early for feedback to see what the technical concerns are
and if we can address them.

[1] https://blog.cloudflare.com/how-we-built-spectrum/

Suggested-by: Jakub Sitnicki <[e-mail]>
Signed-off-by: Tiago Lam <[e-mail]>
---
Changes in v2:
- Amended commit messages and cover letter to make the intent and
  implementation clearer (Willem de Bruijn);
- Fixed socket comparison by not using socket cookies and comparing them
  directly (Eric Dumazet);
- Fixed misspellings and checkpatch.pl warnings on line lengths (Simon
  Horman);
- Fixed usage of start_server_addr() and gcc compilation (Philo Lu);
- Link to v1: https://lore.kernel.org/r/[e-mail]

---
Tiago Lam (3):
      ipv4: Support setting src port in sendmsg().
      ipv6: Support setting src port in sendmsg().
      bpf: Add sk_lookup test to use ORIGDSTADDR cmsg.

 include/net/ip.h                                   |  1 +
 net/ipv4/ip_sockglue.c                             | 11 +++
 net/ipv4/udp.c                                     | 35 +++++++++-
 net/ipv6/datagram.c                                | 79 ++++++++++++++++++++++
 net/ipv6/udp.c                                     |  8 ++-
 tools/testing/selftests/bpf/prog_tests/sk_lookup.c | 67 ++++++++++++------
 6 files changed, 176 insertions(+), 25 deletions(-)
---
base-commit: 6562a89739bbefddb5495c09aaab67c1c3756f36
change-id: 20240909-reverse-sk-lookup-f7bf36292bc4

Best regards,